At present, in ITU-R (International Telecommunication Union-Radiocommunication sector), an IMT (International Mobile Telecommunication)-Advanced system is invited. In 3GPP (3rd Generation Partnership Project), the standardization of LTE-Advanced (LTE-A) which improves system performance is performed while keeping backward compatibility with Rel. 8 LTE (Release 8 Long Term Evolution).
In RAN1 of 3GPP, inter-cell cooperative transmission/reception (CoMP: Coordinated Multi-Point Transmission and Reception) which controls transmission power between many base stations or a transmission base station on the basis of variation in instantaneous interference power is studied, for LTE-A. In order to realize CoMP, as an addition reference signal, a pilot signal (CSI-RS: Channel State Information-Reference Signal) for down space information estimation is studied. The CSI-RS is a reference signal which is used for estimating frequency characteristic information (Channel State Information) of a spatial propagation path of a line (see NPL 1). It is assumed that a base station which supports CoMP transmits a CSI-RS. Here, the base station is a cell or an eNB (enhanced Node-B). The CSI-RS should be transmitted to estimate a line of each cell for CoMP.
FIG. 14 is a schematic view showing when a terminal (UE: User Equipment, mobile station) is given support of CoMP from a plurality of base stations. In order to receive CoMP in a downlink, a terminal 151 should receive CSI-RSs transmitted from a plurality of base stations (cells) 161, 162, and 163, and should accurately estimate space information.
FIG. 15 shows a pattern example (R1-101676) of resource disposition when up to three cells can be multiplexed on four antenna ports as Simulation Assumption agreed on RAN1 #60. FIG. 15 shows resources which constitute one subframe and one RB (Resource Block). In FIG. 15, the vertical axis represents subcarriers (12 subcarriers) of OFDM (Orthogonal Frequency Division Multiplexing) at frequency, and the horizontal axis represents OFDM symbols (14 OFDM symbols of #0 to #13) at time. In FIG. 15, one piece of resource region is one RE (Resource Element). In the pattern example shown in FIG. 15, CSI-RSs of the antenna ports 0 to 3 of a first cell are transmitted with first to fourth subcarriers of an OFDM symbol #10. CSI-RSs of other second and third cells are allocated to other fifth to eighth and ninth to twelfth subcarriers in the same OFDM symbol #10 and transmitted (see NPL 2).
FIGS. 16 to 20 show pattern examples (Patterns 1 to 5) of a plurality of resource dispositions when up to five cells can be multiplexed on eight antenna ports. The examples shown in FIGS. 16 to 20 are pattern examples obtained by slightly correcting the pattern example (R1-100498) (see NPL 3). As in FIG. 15, FIGS. 16 to 20 show resources which constitute one subframe and one RB. In FIGS. 16 to 20, the vertical axis represents subcarriers (12 subcarriers) of OFDM at frequency, and the horizontal axis represents OFDM symbols (14 OFDM symbols of #0 to #13) at time. Five Patterns 1 to 5 can correspond to five cells.
In Pattern 1 shown in FIG. 16, CSI-RSs of the antenna ports 0 to 3 of the first cell are transmitted with the first, second, seventh, and eighth subcarriers of an OFDM symbol #3, and CSI-RSs of the antenna ports 4 to 7 are transmitted with the first, second, seventh, and eighth subcarriers of an OFDM symbol #10. In Pattern 2 shown in FIG. 17, CSI-RSs of the antenna ports 0 to 3 of the second cell are transmitted with the third, fourth, ninth, and tenth subcarriers of an OFDM symbol #3, and CSI-RSs of the antenna ports 4 to 7 are transmitted with the third, fourth, ninth, and tenth subcarriers of an OFDM symbol #10. In Pattern 3 shown in FIG. 18, CSI-RSs of the antenna ports 0 to 3 of the third cell are transmitted with the fifth, sixth, eleventh, and twelfth subcarriers of an OFDM symbol #3, and CSI-RSs of the antenna ports 4 to 7 are transmitted with the fifth, sixth, eleventh, and twelfth subcarriers of an OFDM symbol #10. In Pattern 4 shown in FIG. 19, CSI-RSs of the antenna ports 0 to 3 of the fourth cell are transmitted with the third, fourth, ninth, and tenth subcarriers of an OFDM symbol #5, and CSI-RSs of the antenna ports 4 to 7 are transmitted with the third, fourth, ninth, and tenth subcarriers of an OFDM symbol #12. In Pattern 5 shown in FIG. 20, CSI-RSs of the antenna ports 0 to 3 of the fifth cell are transmitted with the third, fourth, ninth, and tenth subcarriers of an OFDM symbol #6, and CSI-RSs of the antenna ports 4 to 7 are transmitted with the third, fourth, ninth, and tenth subcarriers of an OFDM symbol #13.
In FIGS. 15 to 20, in regard to each resource region divided in an RE unit, a block A (oblique line) is a region where a CRS (Cell-specific Reference Signal) is likely to be transmitted, a block B (dense dot) is a region where a DMRS (DeModulation Reference Signal) is likely to be transmitted, a block C (sparse dot) is a region where a CSI-RS cannot be disposed, and a block D (empty) is a region where a CSI-RS can be disposed. In the block C, the leading three OFDM symbols #0 to #2 are regions where a PDCCH (Physical Downlink Control CHannel) is likely to be transmitted.
As described above, in order that the terminal receives CoMP through a downlink, since it is necessary to receive a CSI-RS transmitted from each base station as an additional reference signal and to accurately estimate space information, the antenna ports of the base stations should orthogonally transmit the CSI-RSs. When multiplexing CSI-RSs while keeping orthogonality, a maximum of five cells of one subframe is considered.